1. Field of the Invention
The present invention relates to a bidirectional optical transmitting/receiving module. More specifically, it relates to a low-cost bidirectional optical transmitting/receiving module as well as an optical transmitting/receiving device having a high-performance de-multiplexing characteristic, and to a method for manufacturing the bidirectional optical transmitting/receiving device.
2. Description of the Related Art
There has been proposed a bidirectional optical transmitting/receiving module that bidirectionally transmits light of two different wavelengths through a single-core optical fiber so as to perform bidirectional communications simultaneously. For example, as shown in FIG. 16 (corresponds to FIG. 1 of Japanese Unexamined Patent Publication 2004-287186 (Patent Document 1)), the bidirectional optical transmitting/receiving module includes: an optical waveguide substrate 101; a first optical waveguide 104 and a second optical waveguide 105 arranged in a V-letter shape on the optical waveguide substrate 101; an end face 102 formed with its cutting face being almost perpendicular to the intersection part (the right end part in FIG. 16) of the first and second waveguides 104 and 105; and a multilayer optical filter 103 provided while being abutted against the end face 102.
Further, this bidirectional optical transmitting/receiving module includes: an optical fiber 107 that is connected to the outer end face of the second waveguide 105; a light receiving part (light receiving element) 106 arranged to face the multilayer optical filter 103; and a light emitting element 108 that is optically connected to the outer end face of the first optical waveguide 104. Reference numeral 111 is an optical resin layer.
First, considering first wavelength light A in the module shown in FIG. 16, the first wavelength light A emitted from the light emitting element 108 to the first optical waveguide 104 is reflected at the multilayer optical filter 103, and is sent out to the optical fiber 107 through the second optical waveguide 105. Then, when second wavelength light B makes incident on the second optical waveguide 105 via the optical fiber 107, the second wavelength light B transmits through the multilayer optical filter 103 and reaches the light receiving part 106 where it is converted to an electric signal and detected.
In this manner described above, bidirectional communications can be performed with a single-core optical fiber by using two kinds of light with different wavelengths. In this case, the multilayer optical filter 103 executes such selective actions (de-multiplexing characteristic of the multilayer optical filter 103) based on a difference in terms of the wavelengths that the first wavelength light A is not transmitted but reflected, and the second wavelength light B is not reflected but transmitted. When the de-multiplexing characteristic of the multilayer optical filter 103 becomes shifted from the ideal characteristic, an interference (cross-talk) is induced when performing transmission and reception of light, such as transmission of unnecessary first wavelength light A. Therefore, the de-multiplexing characteristic is an important characteristic for the performance of the module.
Further, in a bidirectional optical transmitting/receiving module shown in FIG. 17 (corresponds to FIG. 1 of Japanese Unexamined Patent Publication 2002-31748 (Patent Document 2)), a clad layer 203 is provided on an optical waveguide substrate 201, a V-letter shaped optical waveguide 223 is disposed in the center of the clad layer 203 along the optical waveguide substrate 201, and a dielectric multilayer filter 214 is provided to be in contact with the intersection part of the V-letter shaped optical waveguide 223 that is disposed at a top end face, thereby forming a de-multiplexing unit (FIG. 17A). Reference numeral 215 is a solder film. The de-multiplexing unit disclosed in FIG. 17A is stacked on a unit shown in FIG. 17B. Reference numeral 204 indicates an alignment mark.
FIG. 17B shows the other unit (on the right side of the drawing) that holds the de-multiplexing unit described above. These units are combined to form the bidirectional optical transmitting/receiving module.
The unit shown in FIG. 17B is assembled as follows. An alignment area for placing the above-described de-multiplexing unit is provided on the near side on a multimode linear optical waveguide substrate 251, an over clad layer 233 is laminated on the far side on the multimode linear optical waveguide substrate 251, a linear optical waveguide 221a is enclosed inside thereof, and a 1310-nm cutoff multilayer filter 214a is provided at the end face on the far side of the over clad layer 233. Further, a receiving photodiode 210 and a sub-mount 252 are stacked in order by facing the 1310-nm cutoff multilayer filter 214a. 
In the case of FIG. 17, practically, towards the front side of the linear optical waveguide substrate 251 of FIG. 17, the linear optical waveguide substrate 251 itself is extended and an optical fiber and a light emitting element to be engaged with the V-letter shaped optical waveguide 223 are mounted in that extended area. That is, in this case, the other unit (right side of the drawing) including a V-letter shaped groove for fixing the optical fiber is combined therewith to form the bidirectional optical transmitting/receiving module. As in the case of FIG. 16 described above, this case also uses the dielectric multilayer filter 214a that executes selective actions depending on the wavelengths. Reference numeral 207 is a dicing groove, 212 is a recessed part, 215a is a solder film, and 204 is an alignment mark.
Even though the structures of the optical transmitting/receiving modules of the above-described two related techniques are different from each other, the dielectric multilayer filters thereof are assumed to be formed directly on the dicing face at which the optical waveguide substrate is diced because the forming method thereof is not specifically disclosed. In that case, it is often difficult for the dielectric multilayer filter formed on the dicing face to avoid influences of a roughness generated on the dicing face at the time of dicing. Therefore, a sufficient de-multiplexing characteristic cannot be obtained.